1. Field of the Invention
The present invention relates to a fixing device used in an image forming apparatus, such as copying machines and printers.
2. Related Art
A fixing device used in an image forming apparatus is usually provided with temperature detecting means so as to maintain the surface temperature of a heat roller at a predetermined temperature as disclosed in Japanese Patent-Application Publication No. 2003-98899. As such temperature detecting means, there have been proposed contact type detecting means that includes a contact type thermister for detecting the surface temperature while contacting the surface of the heat roller and non-contact type detecting means that includes a non-contact type thermister for detecting the surface temperature without contacting the heat roller.
When the contact type detecting means is used, the thermister causes damages to the heat roller by contacting the same. Therefore, the thermister is usually arranged to contact an end portion of the heat roller outside of a sheet passage portion so that the damages to the heat roller do not affect the quality of printed images. The sheet passage portion of the heat roller indicates a region of the heat roller where a paper sheet contacts when passing by the heat roller.
However, using the contact type detecting means has the following disadvantage. That is, it takes time for heat to transfer in the axial direction of the heat roller. Although the sheet passage portion of the heat roller, which contacts a sheet and melts toner on the sheet, is decreased in its temperature as the sheet contacts the sheet passage portion, a portion of the heat roller other than the sheet passage portion does not decrease in its temperature in the same manner as the sheet passage portion because such a portion does not contact the sheet. As a result, the temperature of the heat roller detected by the contact type thermister differs from the temperature of the heat roller at the sheet passage portion. This makes it difficult to maintain the sheet passage portion of the heat roller at a predetermined temperature.
It has been proposed to switch the surface temperature of the heat roller according to the type of paper sheet. However, it is difficult to precisely control the temperature of the sheet passage portion of the heat roller based only on the detection result of the contact type thermister in contact with the heat roller at the non-sheet passage portion.
If the temperature of the sheet passage portion of the heat roller is lower than the predetermined temperature, then toner on a paper sheet is not completely melted, resulting in a cold offset phenomenon where the toner falls off the paper sheet. On the other hand, if the temperature is higher than the predetermined temperature, then a hot offset phenomenon in which the melted toner adheres to the heat roller and a phenomenon in which the paper sheet winds around the heat roller occur.
On the other hand, the non-contact type detecting means includes one that detects the temperature using infrared rays and one that detects radiation heat. Because the non-contact type detecting means can detect the temperature of the heat roller without contacting the heat roller, the non-contact type detecting means can be disposed to the side of the sheet passage portion of the heat roller without casing damages to the heat roller. Therefore, the non-contact type detecting means can detect the temperature more precisely than the above-described contact type detecting means. However, the non-contact type detecting means has the following disadvantages.
The non-contact type detecting means using the infrared rays includes a protection member for preventing the effects of air currents or atmospheric motions occurring near a detection element on temperature detection, causing the detecting means to have a large size and a large number of components. This makes it difficult to manufacture detecting means at reduced cost.
The non-contact type detecting means using the radiation heat usually includes a detection element, which is the same type of detection element as that used in the contact type thermister, disposed to the side of the heat roller while avoiding direct contact with the heat roller, for detecting the surface temperature of the heat roller via the air space. Thus, the detecting means has the simple configuration and has advantages in terms of providing compact and less expensive detecting means.
However, because the temperature is detected via the air space, there is time delay between when the surface temperature of the heat roller is changed and when this change in the surface temperature is actually detected by the detection element. As a result, it is difficult to maintain the surface temperature of the heat roller constant, causing the cold offset problems and hot offset problems. In a worse case, paper jams occur due to a paper sheet winding around the heat roller.